Trap one in desert (4)
I believe the answer is:
gobi
'desert' is the definition.
(I know that Gobi is a desert)
'trap one' is the wordplay.
'trap' becomes 'gob' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'one' becomes 'i' (Roman numeral).
'gob'+'i'='GOBI'
'in' is the link.
Can you help me to learn more?
(Other definitions for gobi that I've seen before include "Desert, cauliflower in Indian cuisine" , "Plain" , "Desert in China/Mongolia; cauliflower (Ind.)" , "Mongolian expanse" , "Desert of central China" .)